RE: filmscanners: LED Illumination for Film Scanners

["Austin Franklin" <darkroom@ix.netcom.com>]

> 4. Ability to control intensity of each color
> illuminant separately -- eg., the "Analog Gain"
> control in NikonScan.

The Leaf does that by using three scans, and controlling each scan...

> What I don't yet understand is how the illuminant
> is evenly distributed over the film width, or how
> the uniformity of a LED illuminant compares with
> cold-cathode or fluorescent light sources.

Yep, that was my main concern, but not between LEDs so much as just for one
LED unto it self!  There is also space between LEDs, so I just don't see how
this can be made to give near the evenness of illumination that you can get
with a cold light source (non-point also).

> However, similar issues exist for cold cathode
> and fluorescent lamps.

Not near as drastically though.  The light is typically diffused...but I
guess you could do that to the LEDs too.

> (And you have to admit, a LED power supply is a
> lot simpler than the high-voltage, high-frequency
> supply needed for fluorescent bulbs.)

And the fact that they run a LOT cooler.  But I do have a question as to
wavelength.

> A further advantage to the LED illumination
> scheme is that it works with a monochrome CCD sensor.

He he, or of the three pass scanner, but I really don't consider that an
advantage in my case ;-)

> The monochrome sensor has one less source of
> non-uniformity, since it doesn't have any
> color filtering over the sensor itself.  (I
> imagine these filters themselves must introduce
> some degree of non-uniformity.)

Yes, but the LED it self isn't going to be uniform even across one LED...

> The Leaf also "works around" this issue by
> doing three scans -- presumably using a different
> filter with each pass.  But the Leaf can't
> control illuminant intensity or spectral content
> the way that the Nikon scanners do.

I don't quite know what you mean by that...  How does the Nikon control
spectral content?  They don't change the color of the LEDs, do they?  The
color is fixed...

The Leaf does control the lamp brightness, and the lamp brightness of a
non-point linear diffuse light source is going to be quite a bit more even
than an array of LEDs...